This invention relates generally to analog-to-digital converters and, more particularly, to high-speed, high-resolution analog-to-digital converters employing superconducting Josephson junctions.
High-performance analog-to-digital (A/D) converters are required in a variety of commercial and military electronic devices. Two of the more important measures of an A/D converter's performance are its speed, as measured by the number of samples converted per second, and resolution, as measured by the smallest increment of change that can be detected in an analog signal. Superconducting technology is particularly well suited to performing high-speed, high-resolution A/D conversion because Josephson junctions, which are the basic switching elements utilized in superconducting electronic devices, possess a unique combination of speed, sensitivity, and periodic response characteristics.
The Josephson junction is a simple bistable switching device having a very thin insulating layer sandwiched between two superconducting electrodes. When current supplied to the Josephson junction is increased above the critical current of the junction, the device is switched from a superconducting zero-voltage state to a resistive voltage state. The resistive voltage state, in which the voltage drop across the device is equal to the energy gap of the superconductor material, is switched off by reducing the current supplied to the junction to about zero. Because this switching operation can occur in as little as a few picoseconds, the Josephson junction is truly a high speed switching device. In a superconducting A/D converter, one or more of the Josephson junctions are combined with one or more inductive loads to form a logic circuit called a SQUID, or Superconducting Quantum Interference Device.
The characteristics of the SQUID that provide the basis for superconducting A/D conversion can best be explained with reference to a single-junction SQUID. The single-junction SQUID, which is simply a Josephson junction connected across a inductance to form a superconducting loop, exhibits a periodic and multi-valued relationship between the current injected into the inductance and the magnetic flux in the loop. The magnetic flux in the loop increases by a small quantum whenever the current increases by a small and precisely repeatable increment. This quantum of flux gives rise to a small but measurable voltage pulse across the junction. When the current is decreased by a like increment, the magnetic flux in the loop decreases by the small quantum, and a corresponding voltage pulse of the opposite polarity is produced across the junction.
In order to convert a signal from analog to digital form using the single-junction SQUID, the analog signal is introduced into the SQUID as a varying current. Each time the current increases or decreases by a predetermined amount, a measurable voltage pulse is generated across the junction. In this manner, the SQUID functions as a quantizer, with the resultant pulses being detected and counted in one or more binary counters. The principle advantages of this superconducting counting A/D converter are its near perfect linearity and its high sensitivity and speed. The current increment or decrement, which determines the resolution of the device, can be made extremely small. This is because a single flux quantum is only 2.07.times.10.sup.-15 weber and the current increment or decrement is the flux quantum divided by the value of the inductance (measured in henries).
U.S. Pat. No. 4,646,060 to Phillips et al. discloses a superconducting counting A/D converter having a double-junction SQUID quantizer and a bidirectional binary counter comprised of n stages of double-junction SQUID flip-flops, where n is the number of bits of accuracy of the counter. The quantizer generates up-count and down-count voltage pulses of the same polarity on two different output lines, rather than voltage pulses of the opposite polarity on the same output line, as in the single-junction SQUID. The bidirectional binary counter algebraically counts the voltage pulses, increasing the binary count when up-count pulses are received and decreasing the binary count when down-count pulses are received. Superconducting logic circuitry, which includes numerous AND and OR gates, provides the carry and borrow functions of the counter.
Although the Phillips et al. A/D converter has certain advantages, it also has several disadvantages. One disadvantage is that the additional logic circuitry reduces the speed and increases the device count of the A/D converter. An increased device count reduces the reliability of the A/D converter and also increases its cost. Accordingly, there has been a need for a superconducting analog-to-digital converter that does not require this additional logic circuitry. The present invention fulfills this need.